| StalfoS |
Hi all,
Yesterday I managed to score a Mission 70ASA 100w active subwoofer for a lowly $30, only the amp seems to be completely busted, and blows fuses one after the other. I pulled off the panel, and it seems to be a "s53ami" module digital amp, which have a reputation for crapping out.
So I'm thinking of replacing the amp with a lm3886 following an active low pass filter. The transformer that came with it has 33V secondaries but seems to be sort of small-ish for an amp that is supposed to be 100w. Do you think that I should ditch it and find a new one or will it work out okay?
Any suggestions for how I should go about this? Are their better solutions then the lm3886? |
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| BWRX |
| You may want to have a look at some class d modules. |
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| raromachine |
| Wouldn't 33VAC secondaries give something like +/- 46VDC when rectified? And isn't that 4v over the max of the 3886/4780? |
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| Bassterror |
Hi
if your woofer has min 8Ohm, you could parallel 2 TDA7293. |
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| sek |
I'd recommend bridge-tied loading or parallelling of LM3886s according to National's AN-1192.
Or buy a UCD module. BPAs are cheaper, though. :) |
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| richie00boy |
| IMO for just 100 watts class-ab is fine and will be cheaper and DIY. Could you fit something like a P3a in there? |
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| raromachine |
To support richie00boy's comments - I have a paralleled 4780 feeding a cheap 12" 4ohm driver - and at 75% of the output volume from the pre-amp it sits about right with the rest of the speakers in the room. The sub isn't in a box yet - nor does it have the Linkwitz transform I intend to use, so that could all change.
The LM4780 is running +/- 24.5VDC supply which the Overture Design Guide Spreadsheet tells me should give about 105W output power.
With +/- 34VDC the output power (for a parallel setup) raises to 205W - and the size of the heatsink starts to get big too. |
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| BWRX |
| quote: | Originally posted by richie00boy
IMO for just 100 watts class-ab is fine and will be cheaper and DIY. Could you fit something like a P3a in there? |
Class ab would be fine but it may not be cheaper or better than other solutions. http://www.41hz.com/ has some nice low cost kits that could probably be used. UcD/other premade modules would be more expensive but are more or less a drop in solution. |
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| BrianGT |
I measured around 80wrms in lab with my lm4780, with a 2x25vac transformer putting out +/- ~34vdc, with a 4 ohm resistive load box.
If you need much higher power for a sub, I would try again to search for a cheap plate amp.
--
Brian |
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| StalfoS |
Thanks for the responses guys!
I will look into some class-d solutions.. but I don't think it needs to be too powerful. It is 8-inch driver and I have a small appartment. Furthermore the box is about about a 1.5x1.5x1.5 cube -- not too big.
Ideally I would use the existing transformer because it is allready mounted to the inside quite nicely.. but as raromachine pointed out, +/- 46VDC will be a bit much for the 3886/4780's. Also it is not that big/heavy so I worry that the VA rating is not very high. I guess I will have to go transformer shopping.
I figure something in the 70-100 W area would work quite nicely. I don't really have the resources to make PCB's so that's why I was thinking part to part gainclone, but if there are kits available that would be cool too.
What do you suggest?? |
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| Bonicon |
Hi,
Yesterday I brought home a sub (2+1 channel plateamp on it) someone had left for garbage by the curb, a chinese made "Raphaie" unit which I thought would be all ****. However, I was surprised to see a nice toroidal tranny in it, with a pcb attached to a fairly big heatsink, which by the way was tumbling around inside the box as the previous owner seemed to have unscrewed the bolts so it dropped from the plateamp. I don't know if he tried to fix it or what. I can't see any obvious damage to the amp or woofer, but the fuses were blown, so there might be some issues. I didn't have any replacement fuses so I couldn't check it further last night.
Anyways, there where 2 LM4766T chips in this amp, but I cannot comment on their sound yet. Just thought you might be interested to see what is used in a commercial sub, albeit one with no noble pedigree (this sub should be real cheap, I found it online for around 6000 yen, but still some parts are really robust and not cheap). It should be fun to play around with this sub though, even more so as all it cost me was a little effort to bring it home. I think it could work well with a 2-channel and sub TV-setup which is my intended use once I bring it back to life. |
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| AndrewT |
| quote: | Originally posted by raromachine
To support richie00boy's comments - I have a paralleled 4780 feeding a cheap 12" 4ohm driver - and at 75% of the output volume from the pre-amp it sits about right with the rest of the speakers in the room. The sub isn't in a box yet - nor does it have the Linkwitz transform I intend to use, so that could all change.
The LM4780 is running +/- 24.5VDC supply which the Overture Design Guide Spreadsheet tells me should give about 105W output power.
With +/- 34VDC the output power (for a parallel setup) raises to 205W - and the size of the heatsink starts to get big too. |
you've mis-read the datasheet. |
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| Alfetta87 |
| quote: | Originally posted by Bassterror
Hi
if your woofer has min 8Ohm, you could parallel 2 TDA7293. |
As far as I know, paralleling two TDA7293 can be done with 4 ohms as well. The more you put in parallel, the lower the impedance can be.
In bridged mode though, min. impedance is 8 ohms.
Cheers,
- dan |
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| Bassterror |
| quote: | | As far as I know, paralleling two TDA7293 can be done with 4 ohms as well. |
Mine works with a 30V Transformer, yes.
But with a 33V Transformer... a very hot setup. |
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| AndrewT |
Hi,
is that centre tapped AC voltage?
30-0-30Vac sounds very high.
33-0-33Vac is more like a chipbreaker. |
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